EXTREMEPUZZLE BOOKS

Extreme Sudoku 2, puzzle 127: step-by-step solution

Step through the whole solution, or skip to the hard parts. Blue digits are placed by the walkthrough; red strike-through marks a candidate being ruled out.

Puzzle
Sudoku, number 127 of 150 in Extreme Sudoku 2
Difficulty
Expert 7 (expert and beyond), from the final, hardest third of the book
Key technique
XY-Chain
Hard steps
15 of 38 steps need an advanced or expert technique

Blue digits are placed by the walkthrough. Red strike-through marks a candidate being ruled out. Cells are named by row and column: R3C5 is row 3, column 5. Arrow keys step too.

The key technique: XY-Chain

A chain of two-candidate cells, each linked to the next. Whichever way the start cell goes, one end of the chain holds the digit, so cells seeing both ends cannot.

Other advanced and expert techniques in this solve: Nice Loops/AICs (×3), ALS-XZ, Cell Forcing Chains, Finned Fish, Sue de Coq, W-Wing, WXYZ-Wing, X-Wing/Swordfish.

Every move above the basics is listed on the sudoku techniques page, each with a one-sentence explanation and worked examples from the books.

The hard steps, in text (15)
  1. Step 7 · Finned Fish expert
    Rules out 5 from R6C4.
  2. Step 8 · WXYZ-Wing expert
    Rules out 6 from R9C4.
  3. Step 10 · ALS-XZ expert
    Rules out 8 from R8C5.
  4. Step 11 · Nice Loops/AICs expert
    Rules out 5 from R6C7, R6C8.
  5. Step 12 · Nice Loops/AICs expert
    Rules out 1 from R5C8.
  6. Step 14 · X-Wing/Swordfish advanced
    Rules out 5 from R6C1, R6C3.
  7. Step 15 · Cell Forcing Chains expert
    Rules out 8 from R4C8.
  8. Step 17 · Sue de Coq expert
    Rules out 7 from R1C7, R2C7, R3C7, R6C9; 8 from R4C9, R6C9; 4 from R6C9.
  9. Step 20 · XY-Chain expert
    Rules out 1 from R2C9, R3C9, R6C8.
  10. Step 22 · W-Wing advanced
    Rules out 8 from R9C1.
  11. Step 23 · XY-Chain expert
    Rules out 7 from R3C9.
  12. Step 25 · XY-Chain expert
    Rules out 9 from R5C3, R6C8.
  13. Step 27 · Nice Loops/AICs expert
    Rules out 4 from R5C3.
  14. Step 29 · XY-Chain expert
    Rules out 5 from R9C1.
  15. Step 33 · XY-Chain expert
    Rules out 5 from R9C4.

The book

Volume two: 150 new classic 9×9 grids, none repeated from book 1, ramping from hard to brutal.

150 new puzzles. Expert and beyond. Puzzles by M. J. Bailey. Paperback, 5 × 8 in, 196 pages, £9.99 / $11.99. Every solution in the book carries a QR code to its own walkthrough, so you can check one step without seeing the whole answer.

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More step-by-step solutions: Extreme Sudoku 1 · Extreme Killer Sudoku 1 · Extreme Killer Sudoku 2 · Extreme Sudoku 3 · Extreme Killer Sudoku 3.